drm/ttm: Warn on pinning without holding a reference

Not technically a problem for ttm, but very likely a driver bug and
pretty big time confusing for reviewing code.

So warn about it, both at cleanup time (so we catch these for sure)
and at pin/unpin time (so we know who's the culprit).
Reviewed-by: Huang Rui <ray.huang@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Christian Koenig <christian.koenig@amd.com>
Cc: Huang Rui <ray.huang@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201028113120.3641237-1-daniel.vetter@ffwll.ch

drivers/gpu/drm/ttm/ttm_bo.c

@@ -514,7 +514,7 @@ static void ttm_bo_release(struct kref *kref)
 		 * shrinkers, now that they are queued for
 		 * destruction.
 		 */
-		if (bo->pin_count) {
+		if (WARN_ON(bo->pin_count)) {
 			bo->pin_count = 0;
 			ttm_bo_del_from_lru(bo);
 			ttm_bo_add_mem_to_lru(bo, &bo->mem);

include/drm/ttm/ttm_bo_api.h

@@ -600,6 +600,7 @@ static inline bool ttm_bo_uses_embedded_gem_object(struct ttm_buffer_object *bo)
 static inline void ttm_bo_pin(struct ttm_buffer_object *bo)
 {
 	dma_resv_assert_held(bo->base.resv);
+	WARN_ON_ONCE(!kref_read(&bo->kref));
 	++bo->pin_count;
 }
 
@@ -613,6 +614,7 @@ static inline void ttm_bo_unpin(struct ttm_buffer_object *bo)
 {
 	dma_resv_assert_held(bo->base.resv);
 	WARN_ON_ONCE(!bo->pin_count);
+	WARN_ON_ONCE(!kref_read(&bo->kref));
 	--bo->pin_count;
 }